1. Field of the Invention
The present invention relates to a method for manufacturing an electrophotographic elastic roller for use in image forming apparatuses such as printers and copy machines, and electrophotographic process cartridges.
2. Description of the Related Art
Conventional electrophotographic recording apparatuses will be described below. Such an apparatus incorporates an image forming part in its main unit, and an image is formed by undergoing cleaning, charging, latent imaging, developing, transfer, and fixing processes. An image forming part includes a photosensitive drum which is an image carrying body, a cleaning part, a charging part, a latent image forming part, a developing part, and a transfer part. An image on the photosensitive drum, which is formed at the image forming part, is transferred to a recording material with a transfer member and conveyed, thereafter being heated and pressurized at a fixing part to be discharged as a fixed recording image.
In a printer that utilizes an electrophotographic scheme, the photosensitive drum is uniformly charged by a charging roller and forms an electrostatic latent image by a laser, etc. Next, a developing agent in a developer container is uniformly coated on a developing roller at an appropriate charge by a developing agent coating roller and a developing agent regulating member, and a transfer (development) of the developing agent is performed at a contact part between the photosensitive drum and the developing roller. Thereafter, the developing agent on the photosensitive drum is transferred to recording paper by a transfer roller and is fixed by heat and pressure (a pressure roller and a fixing roller). The developing agent which has remained on the photosensitive drum is removed by a cleaning blade, thus completing a series of processes.
In an electrophotographic apparatus, for example in the case of a developing roller, it is always in pressure contact with a photosensitive drum and a developing agent regulating member. When development is performed, the developing roller and the photosensitive drum, and the developing roller and the developing agent regulating member are in pressure contact with each other with the developing agent being interposed therebetween. The developing agent that is not transferred to the photosensitive drum is scrapped off by a developing agent coating roller and is returned to the developer container again. In the container, the developing agent is stirred and conveyed onto the developing roller again by the developing roller coating roller. Repeating these processes will result in that the developing agent undergoes high stress. Accordingly, for the purpose of mitigating stress to the developing agent, the developing roller is formed of a material having an appropriate elasticity. Further, in the cases of the developing roller and the charging roller, since they rotate constantly in contact with other members, a stable contact state must be maintained and therefore a high dimensional precision as a roller is required. If a stable contact state cannot be maintained, the feed amount of the developing agent may vary and also the pressure distribution of the photosensitive drum may vary thereby affecting the resulting image.
Moreover, in recent years, needs for the conversion to color images and higher quality images of electrophotography have grown and accordingly higher precisions for outer dimensions and thickness (suppression of deflection) of the electrophotographic elastic roller are stringently required. For example, in a contact development scheme, since the developing roller is in contact with the surface of the photosensitive drum as described above, insufficient precisions in the outer dimensions and thickness will cause variations in the nip width and nip force between the photosensitive drum and the roller leading to image defects such as density variations.
Developing rollers for use in such a contact developing scheme are configured to have an elastic roller provided with an elastic layer around a shaft core body. Moreover, as desired, an elastic roller is configured to have a surface layer obtained by coating various resin solution to the outer periphery of the elastic layer to provide surface quality.
Conventionally, it is often the case that molding methods using dies are used to manufacture elastic rollers. Moreover, as the method of forming an elastic layer material on the outer periphery of a shaft core body without using dies; various processes, for example, a spray coating process, an immersion coating process, a roll coating process, a blade coating process, a method of coating in a circular coating tank, a coating process using a circular coating head, and others are being studied.
In these conventional coating methods, to achieve an elastic layer having a thickness of about several millimeters, a high-viscosity elastic layer material is diluted with a solvent, and the elastic layer material is coated with the viscosity thereof being lowered to a level needed for coating work. Thereafter, the solvent used for the dilution of the elastic layer material is removed by, for example, vaporizing thereby forming an elastic layer. However, it is inevitable to repeat the process of overcoating the formed elastic layer with an elastic layer material and the process of removing the solvent, resulting in a very low productivity. Moreover, in the elastic rollers manufactured in this way, since many processes are repeated causing the tolerances in the dimensional precision and the deflection (thickness precision) to build up, it is difficult to control the dimensional precision and the deflection (thickness precision) as an elastic roller.
One method for directly coating a high-viscosity elastic layer material onto a shaft core body is a coating method utilizing a circular coating head (see, for example, Japanese Patent Application Laid-Open No. 2006-293015). In this method, the shaft core body and a ring-shaped coating head are disposed such that a uniform gap is formed between the inner peripheral surface of the coating head and the outer peripheral surface of the shaft core body. This method includes a step of causing non-hardened elastic layer material to be ejected from the coating head while moving the shaft core body and the coating head to form a non-hardened elastic layer around the shaft core body, and a step of hardening the non-hardened elastic layer to form an elastic layer. Moreover, the non-hardened elastic layer material is a non-Newtonian liquid material having a yield stress of not less than 50 Pa and not more than 600 Pa, and a thixotropy index of not less than 2.0 and not more than 6.5. This method allows an elastic layer material to be directly coated onto the outer periphery of the shaft core body with an apparatus more readily to form a favorable and uniform elastic layer having a layer thickness of about several millimeters.
Further, another method, first, grips and fixes a shaft core body at upper and lower end parts thereof, and corrects the inclination between the both end parts of the shaft core body caused by the grip fix of the shaft core body. Thereafter, using a coating head having a circular slit that opens inwardly, the coating head is relatively moved with respect to the shaft core body and a non-hardened elastic layer material having a viscosity of 10 to 5000 Pa·s is ejected from the circular slit to be coated onto the outer periphery of the shaft core body. The method includes thereafter hardening the material to manufacture an elastic roller (see, for example, Japanese Patent Application Laid-Open No 2008-164987). This method allows for the manufacturing of an elastic roller having a certain level of dimensional precision, particularly a good thickness precision of the elastic layer compared with a high dimensional precision required for the enhancement of image quality, etc.
According to those methods described above, since a large number of high precision dies are not required as in the case of a die molding, the cost of the production facilities can be significantly suppressed. Moreover, since the number of process steps is small, downsizing of the production facilities can be realized. This allows the forming of a low-cost and high-precision elastic roller, but further improvements are desired to address the needs for the conversion to color images and higher quality images of electrophotography.